1. Field of the Invention
The present invention relates to a device for processing video signals transmitted on a transmission line such as a magnetic recording/reproducing system in which erroneous codes are likely to be generated.
2. Related Background Art
In a conventional digital video signal reproducing apparatus such as a digital VTR as shown in FIG. 1, a rotary head 10 mounted on a rotary cylinder converts a magnetic signal recorded on a video tape into an electric signal, which is amplified by an amplifier 12 and converted into digital data by a data detection circuit 14 such as a comparator. The output data from the data detection circuit 14 contains errors in some cases. The output data is temporarily written in a memory 16 so that an error correction circuit 18 corrects errors while referring to error correction codes (to be described later) added when the data was recorded. Specifically, the error correction circuit 18 reads the data in the memory 16 to correct errors therein, and writes the corrected data again in the memory 16. The corrected data is read from the memory 16 and converted into an analog signals by a D/A converter 20 to be sent to a video apparatus (not shown) from a video output terminal 22.
In a digital recording system for recording a video signal in the form of a digital signal in a recording medium, an error correction technique is generally used whereby one frame video data is divided into blocks each having a predetermined number of data, and an error correction code is added to each block when the data is recorded, to thereby correct errors at the time of recording/reproducing. In the reproducing operation, a predetermined number of errors generated at the time of recording/reproducing can be corrected by using error correction codes. FIG. 2 shows an example of a block format. In this format, "Sync" is a code representative of the top of a block. "ID" is a code representative of the frame area of each block or symbol. "CRCC (Cyclic Redundancy Check Code)" is an error detection correction code for ID. "Data" represents data in a block, and "P" is a parity for the above-described error correction code.
The data in each block is reproduced from a magnetic tape, and written in a frame memory at an area corresponding to its ID to then restore the original image.
Consider now that after recording a TV program in a video tape, another TV program is recorded in the remaining area of the video tape. Conventionally, the other TV program starts to be recorded regardless of the alignment of recording tracks, so that as shown in FIG. 3, the other program overwrites the previous program at the intersection area of track patterns. Accordingly, the width of the previous track becomes narrow at the intersection area so that data reproduction from the narrow track becomes difficult, resulting in an increased error rate of reproduced signals and in flickers in a reproduced image.
Further, a conventional video signal reproducing apparatus of such digital recording type has a problem in that the quality of a reproduced image is considerably degraded during a reproducing operation other than a normal reproducing operation. For example, during a high speed search reproducing operation, the envelope level of an output from a reproducing head becomes very small periodically as shown in FIG. 4A to that of the reproduced codes and at such a portion they have a lower reliability and a higher possibility of errors.
Also during the normal reproducing operation, if large foreign matters or dust are maintained fixed to a reproducing head over several lines, disturbed images cannot be compensated even with a drop-out compensation circuit, resulting in large flickers in the reproduced image.
The above problems are commonly associated with an apparatus which transmits a digital video signal via a transmission line such as a magnetic recording/reproducing line having a high error occurrence possibility.